1. Field of the Invention
The present invention relates to an electronic magnetic compass which is mounted, for example, on a small ship and which is responsive to the terrestrial magnetism to thereby indicate the azimuth of the ship, and particularly relates to an improvement in cost and performance of the appliance.
2. Description of the Related Art
An electronic magnetic compass responsive to the terrestrial magnetism is used to indicate the course, that is, the azimuth of the bow of a small ship, specifically, such as a pleasure boat.
FIG. 4 is a block diagram showing an example of the conventional electronic magnetic compass. In FIG. 4, the electronic magnetic compass comprises a sensor section 1 for producing an azimuth signal in response to the terrestrial magnetism, an operation unit 25 for performing signal conversion to indicate the azimuth signal, and an indicator 26.
FIG. 5 is a block diagram showing another conventional electronic magnetic compass. In FIG. 5, the electronic magnetic compass comprises a sensor section 1 which is the same as that in the above conventional electronic magnetic compass of FIG. 4, a deceleration mechanism 7, a compass card 8 provided with a scale given over the whole azimuth for indicating a compass direction, an operation section 27 for producing a deviation signal indicating a deviation between an azimuth signal produced from the sensor section 1 and a fed-back indication signal, a driving circuit 28 for amplifying the deviation signal, a driving section 29 for driving the compass card 8 to rotate in accordance with the deviation signal, and an angle sensor 30 interlocked with the compass card 8 so as to feed back the indication signal.
In the conventional electronic magnetic compass having such a configuration as described above, the compass card 8 which follows the azimuth signal produced from the sensor section 1 and on which a scale is provided over the whole azimuth for indicating a compass direction is interlocked with the angle sensor 30 which employs a potentiometer, a resolver, an encoder, or the like, for generating the indication signal with accuracy equivalent to that of the scale of the compass card 8 given over the whole azimuth, so that a follow-up circuit forms a closed loop system. The indication signal produced from the angle sensor 30 follows the azimuth signal produced from the sensor section 1 and the operation is performed so as to make the indication signal and the azimuth signal always agree with each other. On the compass card 8, therefore, the azimuth in accordance with the signal produced from the sensor section 1 is always indicated.
In the conventional electronic magnetic compass as described above. In FIG. 4, the azimuth signal produced from the sensor section 1 is transmitted to the indicator 26 through the operation unit 25, and the azimuth measurement is performed in an open loop system. Accordingly, if an error exists between the sensor section 1 and the indicator 26, the error cannot be corrected, so that the accuracy in measurement is lowered.
Further, in the conventional electronic magnetic compass of the closed loop system as shown in FIG. 5, in order to always perform azimuth indication on the basis of the signal of the sensor section 1, it is necessary that the highly accurate angle sensor 30, such as a potentiometer, a resolver, an absolute encoder, or the like, for producing a signal having resolution of the same degree as that of the sensor section 1 is interlock with the compass card 8 so as to form a closed loop to thereby make the indication signal follow the azimuth signal.
In order to make an absolute encoder generate an indication signal, for example, at every degree, it is necessary to provide a digital code of not less than 9 bits (512) and to provide a device for optically reading a digital code. Accordingly, the structure becomes extremely complicated and the cost becomes high.